This invention relates to a device and a method for bleeding compressor air in a turbofan engine.
It is known to extract compressor air from a compressor of a turbofan engine, in order to vary the mass flow through the compressor and to optimize the latter in respect of its aerodynamic stability. The bleeding of compressor air represents, for example, one possibility for preventing unstable operating states of a low-pressure compressor. These states can occur, for example, when the maximum mass throughput of a high-pressure compressor of the turbofan engine at low speeds is less than that of the low-pressure compressor. This can lead, when the engine is throttled, to flow retention downstream of the low-pressure compressor and to a possible reversal of the flow direction, so-called “surge”.
A device is known from DE 10 2011 101 331 A1 for extracting compressor air from a low-pressure compressor. To do so, the wall of the low-pressure compressor is provided with an opening that issues into an annular duct. The extraction of compressor air can be controlled using a sealing element movable between a closed position and an open position. A device of this type for controllable bleeding of compressor air from a low-pressure compressor is also referred to as a booster bleed valve (BBV) system.
The aim for a turbofan engine must be that the compressor air discharged from the low-pressure compressor is introduced into the secondary flow duct in an area in which the introduction of compressor air in this way does not lead to aerodynamic losses. This risk exists however when the released compressor air is introduced into the secondary flow duct between the fan and the fan stator i.e. in an area in which the air coming from the fan still has a swirl and this swirl has not yet been removed from the flow by the fan stator.
Accordingly, it is provided, for example in US 2008/0063515 A1, that the compressor air released from the low-pressure compressor is guided axially rearwards inside an intermediate casing of the turbofan engine, and not introduced into the secondary flow duct until downstream of the fan stator. This does however have the disadvantage that for the guidance of air inside the turbofan engine in this way, the intermediate casing or other functional components must be adapted in their design, entailing corresponding costs and design effort.
This problem is particularly acute when the fan stator is arranged on the outside of an intermediate casing absorbing structural loads. An intermediate casing of this type absorbing structural loads is also referred to as “IMC” and is used for the attachment of fastening means for suspending the engine from a wing or aircraft fuselage. An intermediate casing of this type is that part of an aircraft engine which is most complex and most time-consuming to manufacture, which is why it is preferably avoided that airflow ducts for the guidance of compressor air are additionally integrated into an intermediate casing of this type.